1. Field of the Invention
The present invention relates to a lens structure to be incorporated in an optical system and, more particularly, to a circumferentially cemented lens unit obtained by cementing a plurality of single lenses at their circumferences, and an optical system incorporating the same.
2. Related Background Art
Optical axes of single lenses are slightly decentered or deviated from each other because the optical surface of each single lens is a curved surface. For this reason, conventionally, the optical axes of single lenses must be accurately aligned and fixed to constitute a high-precision optical system. That is, centering is generally performed to obtain good optical performance.
An optical system is constituted by a plurality of lens groups. Each lens group is constituted by combining single lenses having characteristics corresponding to a specific application purpose. To combine single lenses to obtain a lens group, a cylindrical lens holder having a diameter almost equal to that of each of the single lenses to be combined is used.
As a general method of housing single lenses constituting a lens group in a lens holder, the single lenses are inserted into the lens holder in an order of combination, the decentered or deviated states of the single lenses are adjusted, and the single lenses are then fixed by a press ring or the like.
In this case, the single lenses constituting the optical system are inserted into the lens holder one by one. In this case, a small space is required between the lens holder and each single lens because the single lenses cannot be smoothly inserted into the lens holder unless the diameter of the lens holder is slightly larger than that of each single lens.
When the single lenses are stored in the lens holder one by one, the opposing optical surfaces of the adjacent lenses may be brought into contact. Sometimes, the contact portion of these adjacent lenses may be deviated from an optimal position. For example, as shown in FIG. 5, when the optical surface of at least one single lens 102 is a recessed curved surface, the adjacent single lens tends to be inclined toward the concave lens surface, and the contact portion is deviated from the optimal position.
In insertion of at least two single lenses into a lens holder, assuming that one lens has a concave lens surface and that this concave lens surface is brought into contact with the other lens, the contact portion is deviated from the optimal position due to the presence of a space between each single lens and the lens holder. As a result, decentering occurs in inserting single lenses into a lens holder.
For example, when a concave lens is inserted into a lens holder as the first lens, it can be housed such that the center of the lens holder is aligned with the optical axis of the concave lens. When a second lens 103 is inclined toward a first concave lens 102, deviation occurs with respect to a central axis 104, as shown in FIG. 5 or 6, regardless of the type (i.e., convex or concave) of the second lens 103.
This lens inclination poses a serious problem in a lens required for a high precision. A cumbersome operation is required for an operator to correct this inclination. In some cases, adjustment is impossible, and decentered single lenses may be fixed in a lens holder.
Decentering is caused by insertion of single lenses into a lens holder in addition to decentering of a single lens itself. In some cases, the optical axes of single lenses are considerably deviated from each other, and a lens group having excellent optical characteristics cannot be obtained. In particular, when a concave lens surface is to be cemented to another concave lens surface, it is difficult to avoid decentering. In formation of a lens group using a lens holder, high-precision assembly free from decentering forces the operator to precisely adjust each single lens, and working efficiency and productivity are degraded.